1. Field of the Invention
The present invention relates to a lithium secondary battery which may be used as a power source for retaining data in a memory of an electronic apparatus (e.g., a personal computer) or for driving a portable electronic apparatus (e.g., a portable telephone receiver). The present invention also relates to a cathode composition used for such a battery.
2. Description of the Related Art
As is well known, a lithium secondary battery comprises a cathode dischargeably charged with lithium ions, an anode and an electrolyte which allows migration of lithium ions between both electrodes. The anode may consist of lithium metal, a lithium alloy or any other material which can be releasably doped with lithium ions. Typically, the electrolyte may be a nonaqueous electrolytic solution which is prepared by dissolving a lithium salt in an organic solvent.
Due to the high energy density and the use of an organic solvent, a lithium secondary battery is known to have a problem of generating a large amount of heat under severe conditions. For example, the lithium battery generates heat at the time of compression (e.g., battery crushing under a heavy object), nail piercing (e.g., when erroneously driving a nail into the battery at the time of packaging), internal shorting, exposure to high temperature, or external shorting.
One way to solve such a problem is to provide a porous separator between the cathode and the anode, as disclosed in JP-A-54(1979)-52157 or JP-A-59(1984)-207230 for example. According to this solution, the pores of the separator are closed at the melting point of the separator material due to the fusion thereof, thereby interrupting the ion migration between the cathode and the anode. As a result, the current flow terminates to stop the temperature rise.
As an improvement to a lithium secondary battery incorporating a porous separator, JP-A-5(1993)-74443 discloses an arrangement wherein the separator has an excess portion projecting beyond the edge faces of the cathode and the anode, and wherein the excess portion of the separator is pressed down against the edge faces of both electrodes by an insulating plate which is thermally fusible to the separator. Such an arrangement prevents excess heat generation or thermal runaway which may occur through shorting between the cathode and the anode due to a shrinkage of the separator near the edge faces of both electrodes after the pores of the separator are thermally closed.
However, the prior art lithium secondary battery incorporating the porous separator operates properly for the prevention of excessive heat generation only when the separator is kept in its appropriate state. Therefore, the battery is incapable of preventing excessive heat generation if the cathode comes into direct contact with the anode upon rupture of the separator under crushing of the battery or if both electrodes are shorted via a nail which has penetrated through the separator. It should be noted that excessive heat generation in a lithium secondary battery occurs because the Joule heat generated at the time of shorting causes oxygen to separate from the cathode active substance for reacting with active lithium.
On the other hand, JP-A-7(1995)-78635 proposes the use, in a lithium secondary battery, of an electrolytic solution which contains LiAsF.sub.6 /1,3-dioxolane+tertiary amine. Normally, the tertiary amine prevents polymerization of 1,3-dioxolane. Conversely, when the temperature of the battery rises due to high-temperature exposure or shorting for example, 1,3-dioxolane starts polymerizing to increase the internal resistance of the battery, whereby the current flow decreases and the temperature of the battery drops.
However, the above-described electrolytic solution contains As in LiAsF.sub.6. Therefore, sufficient care needs to be taken in handling the battery for preventing environmental pollution. Further, the electrolytic solution is known to decompose when the battery voltage increases to no less than 4V, so that the candidate materials for the cathode active substance are limited to those which make the charge terminating voltage of the battery below 4V. This is critically disadvantageous in increasing the energy density of the battery.